I've only really played some of the bigger-name RPG's, so my experience is a bit limited.
That said, my least enjoyable experience with an RPG was FF X. Not because it was a bad game, more because I was unbelievably hyped for it and it ended up being crushingly disappointing.
They pretty much took everything I loved about VII - IX and threw it out of the window. The only compliment I could give it is about the battle system - the linearity, horrible characters, poor customisation and patchy voice acting just completely ruined the experience for me.
I also really didn't enjoy Star Ocean 3.